Using 3GPP LTE as an example wireless communication network context, sounding reference signals (SRSs) are known signals transmitted by mobile terminals or other items of user equipment (UE) to an eNodeB. In turn, the eNodeB uses SRSs to generate channel estimates used for uplink scheduling and link adaptation. Additionally, or alternatively, the channel estimates are used in controlling downlink multiple-antenna transmission, especially in case of Time Division Duplex (TDD), where the uplink and downlink use the same frequencies. Staying with the LTE context, the Technical Specification 3GPP TS 36.211 in part defines SRSs. According to that specification, SRSs have the duration of a single OFDM symbol. Further, the Technical Specification 3GPP TS 36.331 specifies the setup of SRS symbols, such as SRS bandwidth, SRS frequency domain position, SRS hopping pattern and SRS subframe configuration. These settings are configured semi-statically, according to signaling conveyed as Radio Resource Control (RRC) information element(s).
In a cell with a certain number of users, the period of SRS transmission is specified by the number of users and the bandwidth of SRS. When a large number of users share the SRS resources, the SRS period can become very large, which means less frequent channel estimation updates. While this large period might be good enough for many users, there might be some users with more time-variant channels, and therefore a need for more frequent SRS transmission.
In LTE Rel-8/Rel-9, resources for transmission of SRSs are reserved on a cell level in a static or semi-static fashion. Users in a cell are then semi-statically configured to transmit SRSs using the resources reserved in the cell. When resources are reserved for sounding, they are taken from the cell and hence from all users in the cell. This is opposite to the principle that each user device “pays” for its own sounding reference signal overhead.
Release 10 of the 3GPP LTE standard introduces Multiple-Input-Multiple-Output (MIMO) transmissions on the UL. The specifications provide that demodulation reference symbols (DMRS) are precoded (through the use of a multi-antenna weighting matrix, referred to as a precoder), whereas antenna-specific SRSs are transmitted without precoding, for use in channel estimation. Conventionally, SRS symbols from all antennas and all users share the same resources within a cell. Hence, there is a need to increase the resources for sounding reference signals in the cell. However, doing so using rel-8/9 principles takes resources from all users in the cell, despite the fact that the number of users needing more resources for sounding, such as users operating with multiple antenna transmission in the uplink, may be very small. To address the required SRS resources, in addition to periodic SRS transmission, aperiodic SRS transmission has been proposed as well. Similarly, aperiodic SRS transmission requires additional SRS resources, which undesirably consumes resources even from those users with no need for further SRS transmission, and therefore is unfair to some users.